1. Field of the Invention
The present invention relates to an optical irradiation head and an information recording/reproducing device using the optical irradiation head.
2. Description of the Related Art
With the progress of information society, the quantity of information is increasing more and more. In response to such an increase in the quantity of information, there is a demand for a very high-density information recording method and a recording/reproducing device based on this method. As a recording method for realization of high-density recording, attention is being given to a near-field optical recording method such that a microscopic aperture smaller than the wavelength of incident light is formed and near-field light generated from this aperture is used to form a beam spot smaller than the wavelength of incident light.
For example, Japanese Patent Laid-open No. Hei 10-206660 discloses an aperture formed at a sharpened end of an optical fiber as the microscopic aperture in the near-field optical recording method. That is, an optical fiber having a sharpened end is covered with a metal film, and the sharpened end with the metal film is partially cut off by a particle beam such as a focused ion beam (FIB) to thereby form the aperture.
As another conventional technique, a method of forming an aperture having inclined surfaces through a flat plate is disclosed in U.S. Pat. No. 5,689,480. In this method, an Si substrate is patterned by a lithography technique, and the pattern is anisotropically etched to form an inverted pyramidal recess so that the apex of the inverted pyramid as the deepest point of the recess in the substrate is exposed to the back surface of the substrate. The exposure of the deepest point of the recess in the substrate may be effected by any known method such as polishing of the back surface of the Si substrate or etching. Further, a method of improving the efficiency of light propagation by depositing metal on the tip of a pointed core of an optical fiber is disclosed in Opt. Rev., Vol. 5, No. 6 (1998) 369-373. Further, the shape of an optical fiber for improving both the beam spot size and the light propagation efficiency is disclosed in Appl. Phys. Lett., Vol. 173, No. 15.
In the conventional method of forming the microscopic aperture by sharpening the end of an optical fiber, deposition for forming the metal film is not uniform, and there is a problem of instability of an etching rate due to the concentration of an etching solution or the material composition of the optical fiber. Further, there are problems in mass production process such as instability of formation of the apex angle of the conically sharpened end of the optical fiber and difficulty of control of cutting of the sharpened end by FIB. Further, it is difficult to form a multihead because of the use of an optical fiber.
On the other hand, the conventional method of forming the microscopic aperture by etching the semiconductor substrate has various problems in fabrication process such as instability of an etching rate to an aperture size of tens of nanometers, instability of an aperture size due to nonuniformity of the thickness of the Si substrate in relation to a constant etching amount, and instability of the shape of an etching portion due to deviation of crystal orientation in cutting of the semiconductor substrate. Further, since the inverted pyramidal shape is determined by the crystal orientation inherent in the semiconductor substrate, the angle of the inverted pyramid cannot be controlled to a desired optimum angle in some case. Further, the consumption of the materials used in the process is increased because of many steps of separation and dissolution of the substrate, causing an increase in cost.
At the microscopic aperture smaller than the wavelength of incident light, a plasmon is produced along the aperture edge perpendicular to the polarization direction of the incident light, and the plasmon radiates an electric field as if it were a new light source. As a result, the beam spot size, 1/e2, becomes greater than the aperture size, and the beam spot size increases with an increase in distance from the aperture. Moreover, the conventional near-field optical irradiation head cannot be applied to a system of reading information by a magnetic sensor head such as an optically assisted (thermally assisted) magnetic recording system supporting high-density recording.
In a conventional manufacturing method, the near-field optical irradiation head and the magnetic sensor head must be individually manufactured, and the alignment of these heads is required. In particular, the alignment accuracy between the microscopic aperture and the magnetic sensor head must be set to a track pitch or less. The track pitch in high-density recording will become 0.1 μm or less in the future. However, it is difficult to align the near-field optical irradiation head and the magnetic sensor head with such a high accuracy.
In the conventional technique for improving the light propagation efficiency by using an optical fiber, the electric field of the incident light is concentrated at a certain position by a lens effect on a conical surface inside the optical fiber according to the conical shape of the tip surrounded by the deposited metal. In Appl. Phys. Lett., Vol. 173, No. 15, an aperture surface is located on the plane where the electric field is concentrated, so as to reduce the beam spot size and improve the propagation efficiency. This method is effective, but a very high working accuracy is required, so that there is a problem in working process as similar to the above.